1. Field of the Invention
This invention relates in general to electrically powered mowers and relates more particularly to such a mower with the ability to maintain a constant speed on inclines and declines.
2. Background of the Invention
Mowing golf courses requires a precise mower designed and engineered to ensure a closely cropped and consistently cut green. In addition, because of the time required to grow a mature green and the cost associated with starting and maintaining a golf course, it is extremely important that the greens mower operate properly and not damage the green.
Golf course greens maintenance equipment traditionally has utilized internal combustion engines. A number of greens mowers are known in the art. Some are discussed in U.S. Pat. Nos. 3,429,110, and 4,021,996. Riding greens mowers with multiple or gang cutting units are the subject of U.S. Pat. Nos. 3,511,033, 3,668,844, 4,866,917, and 5,042,236. The mowers discussed in these patents all rely on an internal combustion engine as a source of power, and a complex drive mechanism or hydrostatic system for supplying power to the ground engaging wheels and the reel mowing units. However, there are a number of problems associated with the use of a golf course vehicle incorporating an internal combustion engine. First, fuel or hydraulic fluid can leak from the vehicle onto the green and damage the grass. Furthermore, it is inconvenient to service internal combustion engines, and it is inconvenient and hazardous to obtain and store the necessary fuel. Finally, internal combustion engines are a source of both noise and air pollution, and many U.S. cities have recently enacted noise and air pollution prevention statutes that severely limit the time of day and the duration that internal combustion golf course vehicles may be operated.
Taking into consideration the above noted risks and concerns associated with internal combustion engines, battery-powered mowers have become a viable alternative to conventional internal combustion powered greens mowers. Historically, most electric vehicles have utilized series motor designs because of their ability to produce very high levels of torque at low speeds. The current electric greens mowers, however, have a distinct disadvantage relative to internal combustion engine mowers: since they operate by electric motor, their speed varies on inclines and declines that are greater than about 5 degrees. Such a variation in speed is undesirable because the cutting reels are typically operating at a constant speed. A reel mower is designed to have a certain frequency of clip, wherein a reel blade passes across the bedknife at a certain rate as the machine moves forward. For example, if the height of cut is 0.250 inches, a reel blade will pass the bedknife every 0.250 inches of forward travel. Therefore, since riding Greens Mowers typically have fixed reel speed, any increase in ground speed will cause the machine to move forward farther than what is optimal and cause an uneven, wavy cut that is called marcelling.
Two of the main goals of any type of riding greens mower, whether electrically powered or otherwise, are maintaining the desired cutting height and sustaining an even quality of cut provided by the reels. The ability of the cutting reels to maintain the desired quality of cut depends upon the consistency of the ground speed of the mower. Those skilled in the art will appreciate that variations in terrain and turf conditions affect the ground speed and the quality of cut of an electric greens mower such as disclosed in U.S. Pat. No. 5,406,778.
Internal combustion or hydraulically powered greens mowers incorporate dynamic braking to ensure that the mower maintains a constant ground speed. Dynamic braking is a technique in which retarding force is supplied by the same device that originally was the driving motor. Such braking occurs in an hydraulic system when the operator releases the acceleration pedal. The hydraulic circuit builds pressure internally, creating the braking action, and the motor turns relative to the speed of the hydraulic pump and not faster. The engine, therefore, acts as a brake.
In an electrically powered greens mower, however, dynamic braking, as previously described, is not possible. Rather, when the operator releases the pedal, the voltage applied to the motor drops and the motor enters into a free wheel mode, where the revolution of the electric motor is not inhibited as in the hydraulic motor. As long as the electric mower is coasting on a decline, the electric motor will continue to increase speed. When the mower reaches level ground or an incline, the electric motor will begin to slow gradually until it stops, thus stopping the mower. When operating a mower using this sort of electric motor, the operator must constantly monitor and adjust the ground speed. Those skilled in the art will recognize that the varying speed of the mower is a detraction from using the electrically powered greens maintenance devices of the prior art since the relationship between ground speed and reel speed is not always constant, and marcelling will often result.
Accordingly, there is a need for an electric greens mower that includes a constant speed control mechanism that will prevent the electric motor from entering a free wheel mode, minimize marcelling, and improve the quality of the cut provided by the electric mower.